When the extra rear channel is present but unused, the
s->channel_order_tab[] value for that channel is -1. The QMF can be
skipped for the extra channel, and doing so avoids an out-of-array read
on s->samples_chanptr[].
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>